I can get yonkyo on people quite easily but would strongly disagree that the nerve pain works on everyone. At the end of the day pressure must be applied to the nerve. I have found that it is difficult to penetrate the fat on overweight people, and thus difficult to get yonkyo on them.

I also think that some people are less suceptible (possibly due to pain resistance) - there is a similar nerve near the shin bone which you can get on pretty much anyone, however for some people it doesn't have much effect.